what is the difference to call performSelectorOnMainThread() between in viewDidLoad() and in eventmethod()? (ios)

//viewcontroller.m
 
 -(void)viewdidLoad
 {
    self.theOneViewController= [[TheOneViewController alloc]init];
   [contentsView addSubview:self.theOneViewController.view];
 }
 
 
 //theOneViewController
 
 - (void)viewDidLoad
 {          .
            .
            .
        //UI WORK
            .
            .
     //LONG WORK  
 [self performSelectorOnMainThread:@selector(initAppList) withObject:nil waitUntilDone:NO]; 
 

}

this code, view display UI WORK before LONG WORK is end.So I can have a thread effect.

//viewcontroller.m
 
 -(void) buttonPressed:(id)sender    -> event method
 {
  self.theOneViewController= [[TheOneViewController alloc]init];
  [contentsView addSubview:self.theOneViewController.view];
 }
 
  //theOneViewController
  - (void)viewDidLoad
  {         .
            .
            .
       //UI WORK
            .
            .
   //LONG WORK 
   [self performSelectorOnMainThread:@selector(initAppList) withObject:nil waitUntilDone:NO]; 
  }
 

In this code, view display UI WORK after LONG WORK is end.
So I can't have thread effect. why?
And I use (performSelectorInBackground:withObject:) instead of (performSelectorOnMainThread withObject:waitUntilDone:) . but this is slower than not using thread.

I want to have thread effect in event method call.
Is there a good way? help me please!

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